A towing vehicle-trailer combination may include a generating arrangement (FBM) for generating at least one brake request signal, an electronic towing vehicle brake control unit associated with the towing vehicle for generating brake pressure setpoint value signals for at least one electro-pneumatic pressure control module of the towing vehicle as a function of the brake request signal, a coupleable data link between the towing vehicle and the trailer, and at least one electro-pneumatic pressure control module associated with the trailer for controlling a trailer brake pressure.
Such an electronically controlled brake system is discussed for example in the Technical Information of the Robert Bosch GmbH “Druckluftanlagen für Nutzfahrzeuge, Grundlagen, Systeme und Pläne” [“Compressed Air Systems for Commercial Vehicles, Principles, Systems and Plans”]. In an electronically controlled brake system, normally a purely pneumatic subordinate brake circuit has superimposed on it an electronic overriding control circuit, which suppresses the pneumatic control system, which forms a backup brake circuit in the event that the electrical or electronic brake circuit fails.
Depending on the option, the brake pressure is controlled by axle or by wheel. Systems having an electronic stability program (ESP) require a wheel-specific pressure detection. In the electrical brake system, an error detection triggers a selective shutdown of the affected functions or components.
The braking value sensor is normally made up of a foot brake module, in which two redundant sensors detect the driver's braking command. They transmit the measured value to the central towing vehicle brake control unit, which from this calculates brake pressure setpoint values as electrical signals. These electrical signals are input into a data bus, the brake CAN. On the other hand, in a manner analogous to a conventional foot brake valve, one to two pneumatic control pressures are controlled in accordance with the pedal activation on the part of the driver.
The pressure control modules form the interface between the electronic brake system and the pneumatic braking force. They convert the brake pressure setpoint values transmitted via the data bus into pneumatic pressures. The conversion is performed using proportional solenoid valves or an inlet-outlet solenoid valve combination. An integrated pressure sensor measures the controlled brake pressure in order to implement a brake pressure control in a closed brake circuit. An electrically activated backup solenoid valve blocks the pneumatic control pressures of the foot brake module so as to allow for a non-influenced electronic pressure control.
The pneumatic control pressure to the trailer is likewise controlled electronically via a trailer control module. The electronic trailer control module allows for the pneumatic trailer control pressure to be controlled in accordance with the electrical setpoint value signals of the towing vehicle brake control unit and is constructed in a similar manner as a pressure control module. At the output of the trailer control module there are pneumatic coupling heads for brake pressure and control pressure that are coupleable with complementary coupling heads on the trailer side and are there fed to a trailer brake valve, which forms from this the pneumatic control pressure for the pressure control modules of the trailer in the event of a backup.
In addition to combinations of towing vehicles equipped with electronic brake systems (ELBS) and trailer vehicles that are likewise equipped with such an electronic brake system, it must also be possible to couple such trailers to the towing vehicle as are equipped merely with a purely pneumatic brake system. For this reason, the output pressure generated by the trailer control module is not only used as backup pressure for a trailer having an electronic brake system (ELBS), but rather also as control pressure for the purely pneumatically acting brake system of a conventional trailer. Conventional as well as electrically braked trailers normally have an antilock braking system (ABS) and/or an automatic load-dependent braking force distribution (ALB), in which the braking forces are adapted to the currently prevailing axle loads. Additionally, an electronic stability program (ESP) may be implemented in the control unit of the trailer and/or in the control unit of the towing vehicle such that there exists a relatively high number of variants.
The interplay between the towing vehicle and the trailer is regulated by legislation with the aid of a compatibility diagram. But even within the legal limits, the interaction of certain vehicle combinations may result for example in an unevenly distributed wear between the brakes of the towing vehicle and those of the trailer or may result in the vehicle combination jackknifing during a braking operation. For this reason, in known electronic brake systems (ELBS), the coupling force of semitrailer trains is adjusted to the vertical load borne by the semitrailer tractor and in truck-trailer combinations is adjusted to zero (coupling force control). If the wheel speed from the trailer vehicle is available, then it is possible to balance the braking forces additionally by considering the differential slip between the towing vehicle and the trailer (differential slip control).
In known trailers that are equipped with an electronic braking system, the electrical signals for the brake pressure setpoint values controlled by the towing vehicle brake control unit are transmitted via the trailer CAN to an electronic trailer brake control unit. The brake pressure setpoint values generated by the towing vehicle brake control unit are then corrected or adapted already in the towing vehicle brake control unit and/or in the trailer brake control unit according to one or several of the above-mentioned control systems and are input as electrical brake pressure setpoint value signals into the pressure control modules of the trailer. For this purpose, characteristic curves are stored in a control unit memory which describe the dependence of the input brake pressure setpoint values on trailer-specific boundary conditions and on the control specifications. These characteristic curves are ascertained in tests and are specified at the factory by the trailer manufacturer.
The problem with this, however, is that the communication of the individual systems in the towing vehicle on the one hand and in the trailer vehicle on the other hand is normally limited, which makes the adaptation to different conditions of use and boundary conditions more difficult such that the above-mentioned control functions are sometimes carried out only inadequately. On the other hand, the compatibility requirements between the towing vehicle and the trailer, which postulate for example a compatibility of an electronically braked towing vehicle with a purely conventionally pneumatically braked trailer, severely limit the functionality of the braking device.